The wear resistance and surface roughening resistance of a roll are important properties determining the productivity of rolling. Poor wear resistance causes a roll surface to be prematurely worn, resulting in rolled products with deteriorated dimensional precision. Also, when a roll surface is unevenly worn and roughened by contact with a work, a back-up roll, etc., such surface roughness is transferred to a work surface, resulting in a work with deteriorated appearance. To prevent these problems, the roll should be exchanged frequently, resulting in frequent stop of a rolling operation and thus decrease in the productivity of rolling factories, cost increase by the grinding of a roll surface, and the reduction of a roll life by the increased grinding of a roll surface.
Seizure resistance is also important for the roll. Poor seizure resistance causes a work to be seized with the roll by heat generated in the roll bite, etc. during rolling, failing to conduct normal rolling. Particularly in a downstream stand at a finishing train of a hot strip mill, overlapped or folded end portions of a work are rolled for some reasons, causing an accident called “overlapping.” In this case, if the roll has poor seizure resistance, the work may be seized with the roll and wound around the roll body, inevitably stopping rolling. If rolling continues with the work seized with the roll, a rolling load is concentrated in a seized portion, generating cracks, from which spalling, etc. occurs to cause fracture.
In general, a harder roll has higher wear resistance. A high-speed steel roll material contains high-hardness carbides of alloying elements, such as MC, M2C, M6C, M7C3, etc. Among the alloying elements, particularly V and Nb form extremely hard MC carbides having Vickers hardness Hv of about 2400-3200, remarkably contributing to improvement in wear resistance. However, because MC carbides comprising V and Nb have relatively small specific gravities, when a melt containing large amounts of V and Nb is centrifugally cast, the MC carbide particles are centrifugally segregated inward.
JP8-60289A discloses a centrifugally cast, solid or hollow, composite roll comprising an outer layer having a composition comprising, by mass, 1.0-3.0% of C, 0.1-3.0% of Si, 0.1-2.0% of Mn, 2.0-10.0% of Cr, 0.1-10.0% of Mo, 1.0-10.0% of V, 0.1-10.0% of W, Mo+W≦10.0%, and the balance being Fe and impurities, and an inner layer of cast iron or steel. This reference describes that when V exceeds 10.0% by mass, light carbides are segregated toward an inner surface by centrifugal casting, with small amounts of carbides remaining at an outer surface of an outer layer to be used for rolling. This phenomenon tends to occur when MC carbides are primarily crystallized from a melt. Because primarily crystallized MC carbides have specific gravities of about 6 g/cm3, lighter than the melt having a specific gravity of about 7-8 g/cm3, they move toward an inner surface by a centrifugal force. This reference thus describes that MC carbides are segregated near boundaries with the inner layer, resulting in poor bonding strength between the outer layer and the inner layer.
To prevent segregation due to centrifugal separation, JP9-256108 A proposes a hot-rolling tool steel having carbides with larger specific gravities. This hot-rolling tool steel has a composition comprising by mass 3.5-5.5% of C, 0.1-1.5% of Si, 0.1-1.2% of Mn, 4.0-12.0% of Cr, 2.0-8.0% of Mo, and 12.0-18.0% of V, 8.0% or less of Nb, the balance being Fe and inevitable impurities. JP 9-256108 A describes that although VC having a small specific gravity is segregated by centrifugal casting, the addition of Nb in an amount satisfying the condition of 0.2≦Nb/V prevents the segregation of carbide by centrifugal separation because it forms composite carbide of (V, Nb)C having a large specific gravity. It has been found, however, that the addition of such a large amount of V together with a relatively large amount of Nb causes production problems such as insufficient melting, etc.
Further, the formation of an inner layer inside the outer layer containing such large amounts of alloy components causes insufficient melting problems such as voids between the outer layer and the inner layer, the segregation of carbides, etc., and the deterioration of the toughness of the inner layer due to the migration of large amounts of alloy components from the outer layer to the inner layer.
JP 9-209071 A discloses a composite roll for rolling, which comprises an outer layer made of high-speed cast steel comprising by weight 2.0-3.2% of C, 0.1-2.0% of Si, 0.1-2.0% of Mn, 3-10% of Cr, 2×Mo+W=5-22%, and 3-8% of V, the balance being substantially Fe; an intermediate layer integrally melt-bonded to an inner surface of the outer layer, which comprises 0.8-1.9% of C, 3.0% or less of Si, 2.0% or less of Mn, 6.0% or less of Cr, 5.0% or less of Mo, 5.0% or less of W, and 5.0% or less of V, the balance being substantially Fe; and an inner layer integrally melt-bonded to an inner surface of the intermediate layer, which is made of cast steel material comprising 0.2-0.8% of C, 0.2-3.0% of Si, 0.2-2.0% of Mn, 1.5% or less of Cr, 1.0% or less of Mo, 1.0% or less of W, and 1.5% or less of V, Cr+Mo≧0.3%, and the balance being substantially Fe. This composite roll for rolling has good melt-bonding between the outer layer and the intermediate layer, and between the intermediate layer and the inner layer, thereby avoiding the problem of insufficient melt-bonding in the production of a composite roll comprising an outer layer of a high-carbon material and an inner layer of a low-carbon material by centrifugal casting. Thus obtained is a high-quality, composite steel roll for rolling comprising an outer layer having predetermined wear resistance and an inner layer having predetermined toughness. It has been found, however, that the outer layer of this composite roll has insufficient wear resistance, because of too small amounts of C and V, 0.8-1.9% of C and 5.0% or less of V.
JP 2000-63976 A discloses a composite roll for rolling, which comprises an inner layer made of a structure steel material having excellent toughness is cast on an inner surface of an outer layer made of high-speed cast steel having excellent wear resistance, via an intermediate layer made of Adamite. Because pearlitic transformation occurs in the entire intermediate layer by a heat treatment after casting, there remains a large compression stress on the outer layer surface, suppressing thermal fatigue cracking during rolling and cracking during drawing accident. However, because the matrix of the high-speed steel outer layer is subjected to martensitic transformation and bainitic transformation by a hardening heat treatment step after casting, a radial, residual tensile stress is generated in the boundary between the intermediate layer subjected to pearlitic transformation and the outer layer. When the residual tensile stress of this boundary exceeds the material strength, the outer layer is likely to spall not only by a hardening step, but also by the fatigue of the boundary between the intermediate layer and the outer layer due to rolling stress.
JP 3-254304 A discloses a hot-rolling, composite roll comprising an outer layer having a structure comprising by area 5-30% of MC (granular) carbides and 5% or less of non-MC (non-granular) carbides, the matrix of the outer layer having Vickers hardness Hv of 550 or more. The outer layer in this hot-rolling, composite roll has a basic composition comprising 1.0-3.5% by mass of C, 3.0% or less by mass of Si, 1.5% or less by mass of Mn, 2-10% by mass of Cr, 9% or less by mass of Mo, 20% or less by mass of W, and 2-15% by mass of V, the balance being Fe and impurities. However, this composite roll is produced by a so-called continuous casting method, by which an outer layer is continuously formed around a steel shaft using a high-frequency coil. The continuous casting method suffers from a higher production cost than the centrifugal casting method, and is little adapted for producing a large roll.
JP 4-141553 A discloses a hot-rolling, composite roll comprising an outer layer formed around a steel shaft by a continuous casting method, the outer layer comprising by weight 1.0-4.0% of C, 3.0% or less of Si, 1.5% or less of Mn, 2-10% of Cr, 9% or less of Mo, 20% or less of W, 2-15% of V, 0.08% or less of P, 0.06% or less of S, and 500 ppm or more of B, the balance being Fe and impurity elements, and further comprising any one of 5.0% or less of Ni, 5.0% or less of Co, and 5.0% or less of Nb, and having a structure comprising 5-30% of MC carbide and 6% or more of non-MC carbide by an area ratio, the matrix of the outer layer having Vickers hardness (Hv) of 550 or more. It has been found, however, that when the amounts of V and Nb are increased to improve the wear resistance, primarily crystallized MC carbides float on an outer-layer-forming melt, so that they are unevenly contained in the formed outer layer, resulting in the segregation of carbides.